End Game setup under $500

I am looking forward to start building my first serious rig for home use.

I know for $500 you can't get a true end game rig, but I am looking for suggestion that in your opinion, you can live with for a very long time without feeling anything missing.

For example, I have heard a lot of people swear by Sennheiser HD650 + Bottlehead Crack. But that is like $800. I am looking for a competent headphone with amp that is a perfect pairing under $500.

Some Background: I recently purchased a sennheiser hd518. Its really good. It showed me what a great headphone can do, but its not perfect. Its a bit too smooth in my opinion.

Also I am new to this hobby and have not tried all sound signatures, so am open to most frequency response. Though I would prefer a moderately slow sounding headphone as I listen to music to relax, without sibilance. (setup should not give sibilance overall. Am ok if bright headphones with warm amp or vice versa).